Conventional touch panels are utilized as input-output devices, applicable in various fields, in combination with a display device or unit such as a cathode ray tube (CRT), a liquid crystal display (LCD) or a plasma display panel (PDP). Resistive, capacitive, and acoustic touch panels are presently the dominant types of touch panels in the marketplace. Acoustic touch panels provide a more robust touch surface and greater image clarity than resistive and capacitive touch panels.
Resistive and capacitive touch panels include a resistance layer formed on a substrate. Due to its strength, optical clarity, and low cost, soda-lime glass is generally the preferred substrate material. The resistance layer is essential for the detection of touch position information. In addition, a conventional resistive touch panel includes an overlaying plastic cover sheet. For many applications, such added components to the glass substrate may be susceptible to accidental or malicious damage. Furthermore, these added components degrade the visibility of data and images in a display device as a result of decreased light transmission and increased reflection of ambient light.
In contrast, conventional acoustic touch panels can be advantageously employed in order to insure a robust touch surface and an enhanced display image quality. Because ultrasonic acoustic waves are used to detect coordinate data on input positions, a resistance layer need not be formed on the glass soda-lime substrate and no plastic cover sheet is required. Soda-lime glass is quite transparent and supports propagation of acoustic waves at ultrasonic frequencies. Soda-lime glass is the substrate material of conventional acoustic touch panels. For the end user, such an acoustic touch panel is optically and mechanically little more than a piece of windowpane glass.
Typically, 4% of incident light is reflected off each glass surface resulting in a maximum light transmission of about 92%. Reflection of ambient light reduces image contrast. These reflections are caused by the index-of-refraction mismatch between air and the glass substrate. Decreased light transmission reduces image brightness. These can be important effects when a touch panel is placed in front of a display device having a relatively low luminance (brightness) such as a liquid crystal display. Known methods for reducing reflections and increasing transmission are optical bonding or anti-reflective coatings. These methods address the index-of-refraction mismatch between air and glass. These methods do not improve the inherent transparency of the substrate material itself.
Soda-lime glass is not completely transparent. This is mainly due to color centers caused by iron ion impurities. These iron impurities decrease light transmission and distort the colors of displayed images. These are minor effects relative to, for example, the optical differences between acoustic and resistive touch panels. Nevertheless, improved transmission relative to common soda-lime glass would provide a useful enhancement of the optical advantages of acoustic touch panels.
Display technology is evolving rapidly. This evolution includes introduction and market acceptance of large sized display products. This in turn creates demand for larger touch panels. However, all touch panel technologies encounter problems when scaled to larger sizes. For resistive and capacitive touch panels, it becomes more difficult to maintain sufficient uniformity in resistance layers as panel sizes increase. For acoustic touch panels, the challenge for larger sizes is to assure sufficient signal amplitudes.
For acoustic touch panels, acoustic signals decrease as panel dimensions increase. This signal loss occurs because of the attenuation or damping of the ultrasonic waves as they propagate through the substrate. Thus, large-sized acoustic touch panels may fail to provide sufficient signal-to-noise ratio to reliably determine input positions. Hence there is a need for means to enhance the signal-to-noise ratio for acoustic touch panels. This is all the more true because there are other market pressures for product enhancements that reduce signal amplitudes: lower-cost controller electronics; reduced area reflective arrays; signal-absorbing seals; etc.
Due to the relatively long acoustic path lengths of commercially successful acoustic touch panel designs, acoustic attenuation properties of the glass substrate are particularly important. To understand the need for long acoustic path lengths, consider this first and simplest concept for acoustic touch panels.
Conceptually, the simplest acoustic touch position sensor is of the type described in U.S. Pat. No. 3,673,327. Such touch panels includes a plate having an array of transmitters positioned along one edge of a substrate for generating parallel beams of acoustic waves. A corresponding array of receivers is positioned along the opposite edge of the substrate. Touching the panel at a point causes attenuation in one of the beams of acoustic waves. Identification of the corresponding transmitter/receiver pair determines a coordinate of the touch. The acoustic touch panel disclosed in U.S. Pat. No. 3,673,327 uses a type of acoustic wave known as a "Rayleigh" wave. These Rayleigh waves need only propagate from one edge of the touch panel to the other. However, note that this type of acoustic touch panel requires many transducers, and hence associated cable conductors and electronics channels. This type of acoustic sensor has never been commercialized due to the expense of providing a large numbers of transducers.
Now consider acoustic touch panels that have been commercially successful. Representative of a set of pioneering patents in this field is Adler, U.S. Pat. No. Re. 33,151. An acoustic transducer generates a burst of waves that are coupled into a sheet-like substrate. These acoustic waves are deflected 90.degree. into an active region of the system by an array of wave redirecting gratings. The redirecting gratings are oriented at 45.degree. to the axis of propagation of waves from the transducer. These gratings are analogous to partially silvered mirrors in optics. Acoustic waves after traversing the active region are, in turn, redirected by another array of gratings towards an output transducer. A coordinate of the location of a touch is determined by analyzing a selective attenuation of the received signal in the time domain, each characteristic delay corresponding to a coordinate value of the touch on the surface. Use of the arrays of gratings greatly reduces the required number of transducers, thus making possible acoustic touch panels at commercially competitive prices. On the negative side, this clever use of grating arrays considerably increases the maximum distance acoustic waves must propagate through the substrate.
Signal amplitudes in acoustic touch panels are further decreased by inefficiencies in the scattering process at the grating arrays. Such inefficiencies can be minimized through proper array design. Efficient coherent scattering from the arrays is achieved by orienting the grating elements at a 45.degree. angle and spacing them at integral multiples of the acoustic wavelength. Most efficient use of acoustic energy is provided when the acoustic power "illuminating" the active area is equalized. Known techniques compensate for the tendency for signal amplitudes to exponentially decay as a function of delay time. As described in lines 37 to 41 of column 11 of U.S. Pat. No. 4,746,914, signal equalization can be achieved with a constant wavelength spacing of the grates, i.e., reflecting elements, by providing reflecting elements which vary in height. An alternative method is to selectively drop grating elements to produce an approximately constant acoustic power density over the active area. In this case, the spacing between the grates decreases with increasing distance away from the transducer along the axis of the array. Applying these known methods avoids unnecessary inefficiencies in redirecting the acoustic waves. Nevertheless, the use of grating arrays to twice redirect the acoustic waves inevitably leads to signal losses. This increases the importance of minimum signal amplitude requirements in acoustic touch panel design.
The electronics for commercially available acoustic touch panel products are based on the basic concepts presented in Brenner et al., U.S. Pat. No. 4,644,100. This patent concerns a refinement of the system according to the U.S. Pat. No. Re. 33,151, wherein perturbations of a received signal are determined by comparing the received signal to a stored reference signal profile. By analyzing both the time delays and of the signal perturbations, the touch sensitive system employing acoustic waves is responsive to both the location and magnitude of a touch. Proper operation of the touch system requires a sufficiently large signal-to-noise ratio to avoid ambiguities between signal perturbations due to an acoustic-wave absorbing touch and signal variations due to electronic noise. Electronic noise may be due to fundamental noise from circuit components or due to electromagnetic interference. In recent years, the marketplace increasingly expects a fast touch response from light touches, which requires lower touch perturbation thresholds, and hence increases demand for a higher signal-to-noise ratio.
Further description of such Adler-type acoustic touch panels may be found in the above cited patents as well as in U.S. Pat. Nos. 4,642,423; 4,644,100; 4,645,870; 4,700,176; 4,746,914 and 4,791,416. For each coordinate axis detected, acoustic waves are generated in, e.g., a glass substrate by a transducer containing a piezoelectric element. Thus, a transmitted wave packet is dispersed along the axis of the transmitting reflective array, traverses the substrate and is recombined into an axially propagating wave by another reflective grating, and is directed to a receiving transducer in a direction anti-parallel to the initial transmitted wave. The wave packet is dispersed in time according to the path taken across the substrate. The received waveform is converted into an electrical signal for processing. The time delay of a perturbation of the electrical signal corresponds to a distance traveled by the perturbed component of the wave. Thus, according to this system, only two transducers per axis are required. Typically both X and Y coordinates are measured; this can be done with a total of only four transducers.
Variations of the above acoustic touch panel systems are possible with further reductions in the numbers of transducers. The acoustic wave may be reflected by 180.degree. near or at the edge of the substrate parallel to the axis of the transmission reflective grating and reflected back through the substrate to the reflective array and retrace its path back to the transducer. The transducer, in this case, is configured to act as both transmitter and receiver at appropriate time periods. A second transducer, reflective array and reflective edge are provided for an axis at right angles to allow determination of both coordinates of touch. Still another system provides for a single transducer that produces a wave for detecting a touch on two axes and also receives the wave from both axes. Reducing the number of transducers increases the corresponding acoustic path lengths for a given touch panel size. This increases the signal loss due to acoustic damping within the substrate material.
The touch activating an acoustic touch panel may be due to a finger, gloved or ungloved, or a stylus pressing against the surface. Optionally, the finger or stylus may act indirectly through a cover sheet placed over the glass substrate surface.
There are several modes that ultrasonic waves can take in glass substrates. The mode referred to as a "Rayleigh" wave is of particular interest for acoustic touch panels. Rayleigh waves are essentially confined to a single surface of a sheet of uniform, non-piezoelectric medium of a sufficient finite thickness. Mathematically, Lord Rayleigh calculated the wave function for this mode for a semi-infinite medium. Such waves guided near a surface of a medium of finite thickness are more precisely termed "quasi-Rayleigh" waves, although such waves are generally referred to as "Rayleigh waves" and are so denominated herein. Practical experience with touch panel design and manufacture has shown that about four Rayleigh wavelengths or more is a sufficient substrate thickness to successfully propagate Rayleigh waves.
Other acoustic modes have been investigated for use in acoustic touch panels. U.S. Pat. Nos. 5,260,521; 5,234,148; 5,177,327; 5,162,618 and 5,072,427 disclose the .use of horizontally polarized shear waves and Lamb waves in Adler-type acoustic touch panels. U.S. Pat. No. 5,591,945 discloses further options regarding the choice of acoustic modes in acoustic touch panels. Nevertheless, Rayleigh waves have been, and are expected to remain, the most commonly used acoustic mode in acoustic touch panels. This is due to the relatively high sensitivity of Rayleigh waves to touches and due to their ability to be propagated by a simple surface of a homogeneous medium.
For commercial acoustic touch panels, the frequency of the ultrasonic acoustic waves is around 5 MHz. For acoustic touch panels employing Rayleigh waves, the thicknesses of the soda-lime glass substrates for commercial products to date are in the range from 2 mm to 12 mm. Acoustic touch panel products employing lowest order horizontally polarized shear waves are currently made of 1 mm thick soda-lime glass.
Acoustic touch panels, of the type that has proved to be commercially viable, make clever use of reflective arrays to reduce the number of transducers and electronic channels, and to provide a reliable and accurate time-based analog measurement of touch position. This has proved essential to the commercialization of acoustic touch panels. However, the resulting relatively long acoustic path lengths, along with the losses from two acoustic scatters, leads to small received signal amplitudes. With such small signal amplitudes, it is difficult to assure a sufficient signal-to-noise ratio for reliable signal processing in a touch sensor of the type which transmits ultrasonic acoustic waves in a glass substrate.
Many terms have been used to describe acoustic touch panels: "acoustic sensors", "acoustic touch screens", "ultrasonic touch panels", etc. Unless stated otherwise, all these terms are considered here to be synonyms for a transparent touch sensor which senses touches with ultrasonic acoustic waves and which use reflective arrays of gratings to enable a reduced number of transducers.
There is a need for means to increase signal amplitudes in acoustic touch panels.